Re: [RFC v2 6/9] khugepaged: introduce khugepaged_scan_bitmap for mTHP support

[Usama Arif <usamaarif642@gmail.com>]

On 11/02/2025 00:30, Nico Pache wrote:
> khugepaged scans PMD ranges for potential collapse to a hugepage. To add
> mTHP support we use this scan to instead record chunks of fully utilized
> sections of the PMD.
> 
> create a bitmap to represent a PMD in order MTHP_MIN_ORDER chunks.

nit:

s/MTHP_MIN_ORDER/MIN_MTHP_ORDER/


> by default we will set this to order 3. The reasoning is that for 4K 512
> PMD size this results in a 64 bit bitmap which has some optimizations.
> For other arches like ARM64 64K, we can set a larger order if needed.
> 
> khugepaged_scan_bitmap uses a stack struct to recursively scan a bitmap
> that represents chunks of utilized regions. We can then determine what
> mTHP size fits best and in the following patch, we set this bitmap while
> scanning the PMD.
> 
> max_ptes_none is used as a scale to determine how "full" an order must
> be before being considered for collapse.
> 
> If a order is set to "always" lets always collapse to that order in a
> greedy manner.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  include/linux/khugepaged.h |  4 ++
>  mm/khugepaged.c            | 89 +++++++++++++++++++++++++++++++++++---
>  2 files changed, 86 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> index 1f46046080f5..1fe0c4fc9d37 100644
> --- a/include/linux/khugepaged.h
> +++ b/include/linux/khugepaged.h
> @@ -1,6 +1,10 @@
>  /* SPDX-License-Identifier: GPL-2.0 */
>  #ifndef _LINUX_KHUGEPAGED_H
>  #define _LINUX_KHUGEPAGED_H
> +#define MIN_MTHP_ORDER	3
> +#define MIN_MTHP_NR	(1<<MIN_MTHP_ORDER)
> +#define MAX_MTHP_BITMAP_SIZE  (1 << (ilog2(MAX_PTRS_PER_PTE * PAGE_SIZE) - MIN_MTHP_ORDER))
> +#define MTHP_BITMAP_SIZE  (1 << (HPAGE_PMD_ORDER - MIN_MTHP_ORDER))
>  
>  extern unsigned int khugepaged_max_ptes_none __read_mostly;
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 3776055bd477..c8048d9ec7fb 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -94,6 +94,11 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
>  
>  static struct kmem_cache *mm_slot_cache __ro_after_init;
>  
> +struct scan_bit_state {
> +	u8 order;
> +	u16 offset;
> +};
> +
>  struct collapse_control {
>  	bool is_khugepaged;
>  
> @@ -102,6 +107,15 @@ struct collapse_control {
>  
>  	/* nodemask for allocation fallback */
>  	nodemask_t alloc_nmask;
> +
> +	/* bitmap used to collapse mTHP sizes. 1bit = order MIN_MTHP_ORDER mTHP */
> +	DECLARE_BITMAP(mthp_bitmap, MAX_MTHP_BITMAP_SIZE);
> +	DECLARE_BITMAP(mthp_bitmap_temp, MAX_MTHP_BITMAP_SIZE);
> +	struct scan_bit_state mthp_bitmap_stack[MAX_MTHP_BITMAP_SIZE];
> +};
> +
> +struct collapse_control khugepaged_collapse_control = {
> +	.is_khugepaged = true,
>  };
>  
>  /**
> @@ -851,10 +865,6 @@ static void khugepaged_alloc_sleep(void)
>  	remove_wait_queue(&khugepaged_wait, &wait);
>  }
>  
> -struct collapse_control khugepaged_collapse_control = {
> -	.is_khugepaged = true,
> -};
> -
>  static bool khugepaged_scan_abort(int nid, struct collapse_control *cc)
>  {
>  	int i;
> @@ -1112,7 +1122,8 @@ static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
>  
>  static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>  			      int referenced, int unmapped,
> -			      struct collapse_control *cc)
> +			      struct collapse_control *cc, bool *mmap_locked,
> +				  u8 order, u16 offset)
>  {
>  	LIST_HEAD(compound_pagelist);
>  	pmd_t *pmd, _pmd;
> @@ -1130,8 +1141,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>  	 * The allocation can take potentially a long time if it involves
>  	 * sync compaction, and we do not need to hold the mmap_lock during
>  	 * that. We will recheck the vma after taking it again in write mode.
> +	 * If collapsing mTHPs we may have already released the read_lock.
>  	 */
> -	mmap_read_unlock(mm);
> +	if (*mmap_locked) {
> +		mmap_read_unlock(mm);
> +		*mmap_locked = false;
> +	}
>  
>  	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
>  	if (result != SCAN_SUCCEED)
> @@ -1266,12 +1281,71 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>  out_up_write:
>  	mmap_write_unlock(mm);
>  out_nolock:
> +	*mmap_locked = false;
>  	if (folio)
>  		folio_put(folio);
>  	trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result);
>  	return result;
>  }
>  
> +// Recursive function to consume the bitmap
> +static int khugepaged_scan_bitmap(struct mm_struct *mm, unsigned long address,
> +			int referenced, int unmapped, struct collapse_control *cc,
> +			bool *mmap_locked, unsigned long enabled_orders)
> +{

Introducing a function and not using it probably might make the kernel test bot
and compiler complain at this commit, you might want to merge this with the next
commit where you actually use it.

> +	u8 order, next_order;
> +	u16 offset, mid_offset;
> +	int num_chunks;
> +	int bits_set, threshold_bits;
> +	int top = -1;
> +	int collapsed = 0;
> +	int ret;
> +	struct scan_bit_state state;
> +
> +	cc->mthp_bitmap_stack[++top] = (struct scan_bit_state)
> +		{ HPAGE_PMD_ORDER - MIN_MTHP_ORDER, 0 };
> +
> +	while (top >= 0) {
> +		state = cc->mthp_bitmap_stack[top--];
> +		order = state.order + MIN_MTHP_ORDER;
> +		offset = state.offset;
> +		num_chunks = 1 << (state.order);
> +		// Skip mTHP orders that are not enabled
> +		if (!test_bit(order, &enabled_orders))
> +			goto next;
> +
> +		// copy the relavant section to a new bitmap
> +		bitmap_shift_right(cc->mthp_bitmap_temp, cc->mthp_bitmap, offset,
> +				  MTHP_BITMAP_SIZE);
> +
> +		bits_set = bitmap_weight(cc->mthp_bitmap_temp, num_chunks);
> +		threshold_bits = (HPAGE_PMD_NR - khugepaged_max_ptes_none - 1)
> +				>> (HPAGE_PMD_ORDER - state.order);
> +
> +		//Check if the region is "almost full" based on the threshold
> +		if (bits_set > threshold_bits
> +			|| test_bit(order, &huge_anon_orders_always)) {
> +			ret = collapse_huge_page(mm, address, referenced, unmapped, cc,
> +					mmap_locked, order, offset * MIN_MTHP_NR);
> +			if (ret == SCAN_SUCCEED) {
> +				collapsed += (1 << order);
> +				continue;
> +			}
> +		}
> +
> +next:
> +		if (state.order > 0) {
> +			next_order = state.order - 1;
> +			mid_offset = offset + (num_chunks / 2);
> +			cc->mthp_bitmap_stack[++top] = (struct scan_bit_state)
> +				{ next_order, mid_offset };
> +			cc->mthp_bitmap_stack[++top] = (struct scan_bit_state)
> +				{ next_order, offset };
> +			}
> +	}
> +	return collapsed;
> +}
> +
>  static int khugepaged_scan_pmd(struct mm_struct *mm,
>  				   struct vm_area_struct *vma,
>  				   unsigned long address, bool *mmap_locked,
> @@ -1440,7 +1514,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>  	pte_unmap_unlock(pte, ptl);
>  	if (result == SCAN_SUCCEED) {
>  		result = collapse_huge_page(mm, address, referenced,
> -					    unmapped, cc);
> +					    unmapped, cc, mmap_locked, HPAGE_PMD_ORDER, 0);
>  		/* collapse_huge_page will return with the mmap_lock released */
>  		*mmap_locked = false;
>  	}
> @@ -2856,6 +2930,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
>  	mmdrop(mm);
>  	kfree(cc);
>  
> +
>  	return thps == ((hend - hstart) >> HPAGE_PMD_SHIFT) ? 0
>  			: madvise_collapse_errno(last_fail);
>  }